1. Field of the Invention
The invention relates in general to optical sensors that detect the Faraday effect in crystal media, and more specifically, the class of transmissive optical sensors that rotate the plane of polarization of light traveling through a crystal media that is under the influence of external quantities such as magnetic fields, electrical currents which give rise to magnetic fields, or temperature fluctuations.
2. Related Art
More than 150 years ago, Michael Faraday discovered that when linearly polarized light travels through flint glass that is exposed to a magnetic field, its plane of polarization rotated. This property, now known as the Faraday effect, is widely used in the fiber optic telecommunications field, specifically to prevent reflected light energy from coupling back into a light source and changing source parameters such as frequency or power output. In sensor systems that exploit the Faraday effect, a sensor assembly is placed into a magnetic field. By monitoring the rotation of the incident polarization state, a direct measurement of the magnetic field intensity can be inferred. The relationship governing this phenomenon is best stated as:
"THgr"=VHlxe2x80x83xe2x80x83EQN(1)
where "THgr" is the measured angle of rotation of the field, V is a constant known as Verdet""s constant, H is the applied magnetic field, and l is the optical path length. All materials exhibit the Faraday effect, but the magnitude varies greatly. For example, the Verdet constant for a SiO2 crystal is approximately 3.2exe2x88x924 (deg/cm-Oe), while in ferromagnets such as the value can be on the order of 6.0e+5 (deg/cm-Oe).
When an optical path completely encircles a conductor, a numerical integration can be performed about the optical path, which results in the ability of relating the Faraday rotation directly to the current flowing through the optical path. In this instance, the rotation is related to current I by a form of Ampere""s Law:
"THgr"=V∫Hdlxe2x80x83xe2x80x83EQN(2)
"THgr"=VIxe2x80x83xe2x80x83EQN(3)
Finally, if N optical paths exist around the conductor, the total current in the conductor is                     I        =                  Θ          VN                                    EQN        ⁢                  xe2x80x83                ⁢                  (          4          )                    
Optical fiber is one material that exhibits a small Faraday effect. Based upon this, devices have been known and used for measuring the amount of current flowing through a conductor. By wrapping multiple turns of optical fiber around the conductor and applying Ampere""s Law, the amount of current can be directly measured. Sensitivity is controlled in this fashion: applications requiring higher sensitivity wrap a higher number of turns around the conductor being monitored.
Unfortunately, using optical fiber as a sensor is often impractical in many applications because it is not feasible to interrupt power by disconnecting the conductor, installing the fiber coil assembly, then reconnecting the conductor. Another disadvantage of an all-fiber sensor is that in practical use, the loops that encircle the conductor can be no smaller than 4-5 cm in diameter. Violation of this condition typically results in tremendous temperature sensitivity, which then appears as an undesired rotation of the state of polarization of the desired signal.
Bulk glass is another material that exhibits a Faraday effect. An advantage of the use of bulk glass is that the sensor can be fabricated from materials with a higher Verdet constant, which improves the sensitivity to the influencing magnetic field. These bulk crystals can be annealed, which can release internal stresses, thereby reducing linear birefringence. By themselves, bulk-glass sensors are relatively mechanically stable in both temperature and mechanical handling. Bulk glass can be made relatively inexpensively, which portends well for mass production concepts using these sensors.
Bulk-glass sensors suffer from their own set of limitations. The transducers manufactured from bulk glass are large, relatively on the same order as the all-optical fiber sensors previously described. Bulk glasses are not ferromagnetic, hence their Verdet constants are lower, which restricts their applications to extremely high current measurement. Obtaining multiple circular paths around a bulk-glass arrangement in order to increase the sensitivity of the sensor has been accomplished by some researchers, but there are limitations of using this configuration in applications that experience tremendous temperature fluctuations. Finally, assembly and alignment of bulk-glass sensors has historically been performed by hand, resulting in tremendous labor costs that preclude their widespread use.
Ferromagnetic materials, such as bismuth- and terbium-doped yttrium-iron-garnet (BiTb2Y3Fe5O12) for example, have much larger Verdet constants per unit thickness. This results in a much smaller Faraday rotator to measure a given magnetic field strength, and the outcome is that a whole class of reduced size magneto-optical transducers is enabled. Methods to grow these materials are well established and directly support other markets, specifically optical telecommunications, hence tremendous economies of scale are realized that surpass that of bulk-glass and rival the cost of optical fiber. Packaging of the transducer becomes smaller with the introduction of high-Verdet constant materials, and thus manufacturing costs are significantly less than what is available with all-fiber or bulk-glass designs.
Applications for a reduce-sized magneto-optic transducer continue to grow. For example, the electric utility industry is experiencing tremendous pressures as consumer and regulatory demands upon the industry increase. Consumers, with expanding telecom, data processing, and other energy needs, are demanding xe2x80x9chigh-ninesxe2x80x9d reliability. Utilities are attempting to respond, but are doing so with an antiquated infrastructure that has an average age of 31 years. Regulatory pressures have created large uncertainties in the future ownership of assets, and hence infrastructure improvements have fallen sharply since the mid 1990""s. Additionally, many utilities operate under rate caps and cannot pass costs onto consumers. Not surprisingly, the industry is looking to conserve capital, and is doing so by pushing equipment harder without fully understanding the long-term consequences, as well as deferring maintenance until corrective action is required. Even small percentage changes in distribution system operating efficiencies can result in hundreds of millions of dollars a year in savings. Hence, many utilities are reviewing technologies that can provide efficiency and reliability improvements.
Optical sensor technologies for utility applications promise to deliver lower-cost monitoring solutions to the industry. These technologies provide an entirely new means of measuring electrical current, conductor temperature, voltage, and combustible gasses. When combined with the latest wireless and network topologies, automated data delivery and control is possible, resulting in improved operations. Optical sensory systems which cost less than current state-of-the-art transformer-based systems gives utilities the key to unlocking information by which they can manage their systems much more efficiently, resulting in improved reliability and improved system efficiencies. Furthermore, widespread use of these technologies will result in the immediate notification and location of power faults and outages, potentially saving the utility industry and it""s customers 100""s of millions of dollars in outage costs.
U.S. Pub. No. US2001/0043064A1 to Bosselmann et al. discloses a pi-shaped transmissive polarimetric sensor that is comprised of a polarizer, a sensor element, and an analyzer. An output optical waveguide with a core diameter of at least 100 xcexcm is used. The input light is uncollimated and unfocused. The sensor requires the use of a prism to steer the light from the input fiber to the sensor element, and correspondingly, from the sensor element to the output optical waveguide.
U.S. Pat. No. 6,404,190 (2002) to Itoh et al. discloses a pi-shaped transmissive polarimetric sensor that is comprised of an input optical fiber, a polarizer, a magneto-optical device, a second polarizer comprising an analyzer, and an output optical fiber. Itoh et al. present three embodiments: one that uses spherical or hemispherical lens at each of the fibers, one that omits the polarizers and uses fiber to provide the polarization/analyzer function, and one that uses plastic optical fiber.
U.S. Pat. No. 6,347,885 (2002) to Duncan discloses the use of rare-earth iron garnet magneto-optical films in reflection-type magneto-optical sensors as well as presenting several optical and signal processing topologies that can be used to measure the Faraday rotation due to a changing magnetic field.
U.S. Pat. No. 6,370,288 (2002) to Itoh et al. discloses a pi-shaped transmissive polarimetric sensor that is arranged as a confocal optical system comprised of a input optical fiber, a drum lens, a polarizer, a magneto-optical device, a second polarizer comprising an analyzer, a second drum lens, and an output optical fiber. In this disclosure, one holder contains both the input drum lens and the output drum lens.
U.S. Pat. No. 6,160,396 (2000) to Itoh et al. discloses a pi-shaped transmissive polarimetric sensor that is comprised of an input optical fiber, a first lens, a first mirror, a polarizer, a magneto-optical device, an analyzer, a second mirror, a second lens, and an output fiber. Integrated holders are used to hold the discrete optical components.
U.S. Pat. No. 5,861,741 (1999) to Itoh discloses a linear, transmissive polarimetric sensor that uses multimode optical fiber for the input and output fibers. Only one rod lens is used to couple the energy from the input fiber to the output fiber.
U.S. Pat. No. 5,742,157 (1998) to Ishizuka et al. discloses a pi-shaped transmissive polarimetric sensor that uses one GRIN lens to launch and recover the optical energy in a configuration similar to that disclosed by Itoh et al. in U.S. Pat No. 6,160,396.
U.S. Pat. No. 5,485,079 (1996) to Itoh discloses a linear, transmissive polarimetric sensor comprised of a first lens, a polarizer, a magneto-optical element, an analyzer, and a second lens. Itoh further discloses that this sensor system is a light converging optical system.
U.S. Pat. No. 5,475,298 (1995) to Rogers discloses a reciprocal optical system that dynamically compensates for external perturbations that change the properties of an optical wavefront traveling within an optical fiber.
U.S. Pat. No 5,321,258 (1994) to Kinney discloses a pi-shaped optical sensor unit that is capable of being manufactured as a small sensor package as a result of a novel housing used to align the magneto-optical sensing element with the incoming wavefront. The design eliminates the need for mirrors, prisms, or collimating lenses.
U.S. Pat. No. 5,202,629 (1993) to Seike et al. discloses a pi-shaped magneto-optical sensor having a magneto-optical element, a polarizer, an analyzer, and a substrate to which these elements are bonded. Seike et al. assert that the disclosed sensor is temperature stable from xe2x88x9220xc2x0 C. to +80xc2x0 C., and that this is due to a specialized synthetic resin that is used in the bonding process.
U.S. Pat. No. 5,008,611 (1991) to Ulmer, Jr. discloses a method and apparatus for measuring a target electric current utilizing the Faraday effect in an optical medium.
In xe2x80x9cVibration Compensation Technique for an Optical Current Transducerxe2x80x9d, Opt. Eng. 38(10), October 1999, pp 1708-1714, Niewczas et al. discuss a technique for compensation of vibration-induced noise in a optical current transducer. Their method uses two light sources as well as two photodetectors.
In xe2x80x9cA High-Accuracy Optical Current Transducer for Electric Power Systemsxe2x80x9d Power Delivery, IEEE Transactions on, Volume: 5 Issue: 2 , April 1990 Page(s): 892-898, Ulmer, E. A., Jr. discusses the use of non-45 degree orientations between an incident polarizer and an accompanying analyzer.
In xe2x80x9cA Common-Mode Optical Noise-Rejection Scheme for an Extrinsic Faraday Current Sensorxe2x80x9d, Meas. Sci. Technol. 7 No 5 (May 1996) pp. 796-800, Fisher, N E and Jackson, D A discuss a method to eliminate optical noise induced by fiber optic vibration. Their experiment utilizes a linear, non-reciprocal magneto-optical transducer that places a beam splitter directly onto the sensor assembly.
In xe2x80x9cImproving the Sensitivity of a Faraday Current Sensor by Varying its Operating Pointxe2x80x9d, Meas. Sci. Technol.xe2x80x946 No 10 (October 1995) pp. 1508-1518, Fisher, N E and Jackson, D A discuss a method to improve the performance of a Faraday sensor. Their experiment utilizes a linear, non-reciprocal arrangement of a polarizer, Faraday rotator, and another polarizer, and they discuss the effect of varying the polarization angles between the two polarizers.
It is an object of the invention to provide an improved optical sensor for measuring polarization rotation of optical wavefronts.
It is a further object of the invention to provide an improved optical sensor for measuring the temperature of the sensor element as well as the temperature of the conductor in which the sensor is in contact.
It is a further object of the invention to provide an improved optical sensor that mechanically self-aligns the entire optical path during the manufacture of the device.
It is a further object of the invention to provide an improved optical sensor that can be used in reciprocal mode, e.g., with counter-propagating optical wavefronts simultaneously entering and leaving each fiber.
It is yet a further object of the invention to provide an improved optical sensor that can be easily mass-produced through automation, with minimization or complete elimination of the difficulties of alignment and characterization of the said optical components.
In a preferred embodiment, the invention provides optical sensors that use Faraday rotator materials, e.g., crystalline materials such as rare-earth garnets for example, as to measure magnetic fields, corresponding electrical currents, or temperature fluctuations. The invention may be provided in the form of a fiber optic sensor system preferably comprising an optical fiber coupled to a graded index (GRIN) lens, a polarizer, a Faraday rotator material, another polarizer, another GRIN lens coupled to an optical fiber, an optical detector, and an electronic circuit to analyze the output of the detector.